Photovoltaic devices such as solar cells are capable of converting solar radiation into useable electrical energy. The active solar cell semiconductor material can have a crystalline structure, e.g., single crystalline or polycrystalline silicon, or a non-crystalline structure, e.g., amorphous silicon. Energy conversion occurs as the result of what is well known in the solar cell field as "photovoltaic effect." Two basic steps are involved in the photovoltaic effect. Initially, solar radiation absorbed by the semiconductor generates electrons and holes. Secondly, the electrons and holes are separated by a built-in electric field in the semiconductor solar cell. This separation of electrons and holes results in the generation of an electrical current. A built-in electric field can be generated in a solar cell by, for example, a Schottky barrier. The electrons generated at the metal (Schottky barrier) semiconductor body junction flow towards the semiconductor body.
Cermets, also known as granular metals, are composite materials consisting of finely dispersed mixtures of immiscible metals and insulators, which are known to act as Schottky barriers to N-type single crystal silicon and gallium arsenide, see, J. Appl. Phys., Vol. 45, No. 1, Jan., 1974. However, due to the differences between crystalline and amorphous silicon, IEEE Transactions On Electronic Devices, Vol. ED-24, No. 4, April, 1977, no conclusions can be extrapolated from single crystalline silicon as to the effect of cermets on amorphous silicon. Metals which function as Schottky barriers to N-type single crystal silicon, such as nickel, tend to form ohmic contacts instead to intrinsic or insulating amorphous silicon.
Hydrogenated amorphous silicon solar cells, described in U.S. Pat. No. 4,064,521 to Carlson, herein incorporated by reference, are capable of converting solar radiation into useable electric energy. The hydrogenated amorphous silicon solar cells are fabricated by glow discharging silane (SiH.sub.4) to form a body of hydrogenated amorphous silicon and thereafter evaporating platinum or another high work function metal onto the deposited body of hydrogenated amorphous silicon. The Schottky barrier formed by the evaporation of the metal exhibits inferior diode characteristics immediately after formation and requires annealing at about 200.degree. C. for about 15 minutes. This is a time consuming process which adds to the cost of the final cell.
Attempts to further reduce the cost and speed the processing of hydrogenated amorphous silicon solar cells by sputtering the platinum films result in abnormally high percentages of solar cells with shorts or shunts. Electrical shorts occur when there is a pinhole in the amorphous silicon body and the front and back electrodes are touching. A shunt is the loss of charge in the amorphous body due to imperfect barrier formation or the formation of an ohmic contact by the high work function metal rather than Schottky-like barrier formation. Electrical shorts and shunts either greatly reduce or completely eliminate the efficiency of the solar cell. The back electrode may also render the amorphous silicon solar cell susceptible to shorts or shunts which can further degrade the overall performance and conversion of solar radiation into useable electrical energy. In addition, the problems of solar cell defects which cause shorts or shunts greatly increase with increasing solar cell size.
Thus, it would be highly desirable to find materials which can be applied to amorphous silicon by either sputtering or co-sputtering, to speed the processing, and minimize and localize the effects of electrical shorts and shunts.